C++11 MinGW 4.9.1 shared_ptr 和 const 静态类字段结果 "Undefined reference"

Question

#include <memory>

class CItem
{
  private:
    int m_inner;
  public:
    static const int CAP = 1;
    CItem(int temp) : m_inner(temp) {}
};

typedef std::shared_ptr<CItem> TPItem;

int main()
{
  int tttt = CItem::CAP;
  CItem *temp = new CItem(CItem::CAP);
  TPItem temp2(temp);
  TPItem temp3 = std::make_shared<CItem>(tttt);
  TPItem temp4 = std::make_shared<CItem>(CItem::CAP); //On MinGW there error: "undefined reference to `CItem::CAP'"

  return 0;
}

• 在 Visual Studio 2012 上它工作正常。
• 在 minGW32 4.9.1 上它说 尝试创建时类的静态常量字段的“ undefined reference ” 与 make_shared 共享指针。
• 其他方法:
• 将此字段复制到 int 并创建与此 int 的共享 - 工作。
• 使用 new - work 创建类对象。

我的错在哪里？

Answer 1

这是因为 CItem::CAP 是 odr-used通过 std::make_shared (强调我的):

Informally, an object is odr-used if its address is taken, or a reference is bound to it, and a function is odr-used if a function call to it is made or its address is taken. If an object or a function is odr-used, its definition must exist somewhere in the program; a violation of that is a link-time error.

自 std::make_shared通过引用获取其参数，这算作 odr-use。

这意味着您还需要提供类外定义:

const int CItem::CAP  ;

或者避免使用 odr，例如本例:

TPItem temp3 = std::make_shared<CItem>(tttt) ;

作为引用，C++11 标准草案 3.2 [basic.def.odr] 说:

An expression is potentially evaluated unless it is an unevaluated operand (Clause 5) or a subexpression thereof. A variable whose name appears as a potentially-evaluated expression is odr-used unless it is an object that satisfies the requirements for appearing in a constant expression (5.19) and the lvalue-to-rvalue conversion (4.1) is immediately applied.

CItem::CAP 是一个常量表达式，在除此之外的所有情况下:

TPItem temp4 = std::make_shared<CItem>(CItem::CAP);

立即应用左值到右值的转换。